Mice deficient in interleukin-2 production (IL-2null mice) develop colonic inflammation closely resembling ulcerative colitis in humans. Although this disease is marked by substantial infiltration of the colon by CD8+ and CD4+ T lymphocytes, no function has yet been assigned to these T cell subsets in the development of colitis in the IL-2null mouse. For the present study, we investigated the involvement of T lymphocytes in the onset of colitis in IL-2null mice, and examined the possible role played by cytotoxic T cells. Both lamina propria lymphocytes (LPL) and intraepithelial lymphocytes (IEL) of the colon of IL-2null mice were potently cytotoxic ex vivo in short-term redirected cytotoxic lymphocyte (CTL) assays. In contrast, colonic T cells of wild-type animals showed little or no constitutive cytotoxic T cell activity. Colonic CTL were detectable prior to the appearance of disease in IL-2null animals and CTL activity was confined to the TcR alpha beta, rather than to the TcR gamma delta IEL subset. IL-2null animals crossed with major histocompatibility complex class I-deficient mice [IL-2null x beta 2 microglobulin (beta 2mnull) mice] also developed colitis, which appeared even earlier than in most IL-2null mice. These findings suggest that neither CD8+ IEL nor LPL were causal in the onset of colitis in IL-2null animals. In IL-2null x beta 2mnull mice, an ulcerative colitis-like disease was evident from histological studies and immunohistological staining which showed very large numbers of CD4+ lymphocytes within the intestinal mucosa. Significant ex vivo killing by CD4+ T cells was observed in IL-2null x beta 2null animals, although this required an extended incubation time compared to colonic CD8+ T cells. Peripheral as well as colonic CD4+ T cells in IL-2null and IL-2null x beta 2mnull animals, were activated as judged by their cell surface phenotype (CD45RBlo, L-selectinlo and CD69+). In light of these findings, we propose that infiltrating CD4+, but not CD8+ T cells are central to the inflammation observed in the intestinal mucosa in IL-2null colitis.